A polymer latex obtained by emulsion polymerization generally contains polymer particles which have a particle size of 1 .mu.m or smaller and exist in a state covered with an emulsifier and dispersed and suspended in water. Their particle sizes are too small to permit direct recovery of the polymer by solid-liquid separation. As a conventional method for recovering a polymer from such a polymer latex, it has been widely practiced (1) to use a spray drier so as to directly separate the polymer as a powdery or particulate material or (2) to mix a salt or acid with the polymer latex to coagulate, and then to heat the resultant mixture to solidify the polymer, followed by dewatering and drying to recover the polymer as a powdery or particulate material.
In order to control the particle sizes of the resulting polymer particles, it has also been proposed inter alia (3) to coagulate after dispersing and sphering a polymer latex in a specific solvent (Japanese Patent Laid-Open No. 68285/1977), (4) to add an organic liquid, which is almost insoluble in water and is incapable of dissolving the polymer but is capable of wetting the polymer, to a slurry obtained by coagulating a polymer latex so as to granulate the polymer (Japanese Patent Publication No. 5610/1984), (5) to mix an organic liquid, which is almost insoluble in water and is incapable of dissolving the polymer but is capable of wetting the polymer, with a polymer latex in the presence of a coagulant (Japanese Patent Publication No. 5611/1984), or (6) to disperse droplets of a polymer latex in a coagulating atmosphere by using a spray drier system, thereby semi-coagulating the polymer, and then to solidify the polymer in a coagulating liquid, followed by dewatering and drying (Japanese Patent Laid-Open No. 95905/1981).
Method (1), which uses a spray drier, is however accompanied by the problems that a great deal of drying energy is required because a polymer latex containing a large amount of water is dried as is and a lot of water has to be evaporated, variations tend to occur among the sizes of droplets sprayed and the particle size distribution thus becomes wide, difficulties are encountered in controlling the particle size and/or bulk specific gravity, and a high initial cost is required.
Method (2) featuring coagulation and solidification involves the problems that, when a conventional coagulation and solidification apparatus of the tank or column type is used, the resulting particulate polymer has a broad particle size distribution and fine particles of smaller particle sizes are hence mixed in a large proportion to impair the handling characteristic of the particulate polymer, and especially, fine particles of about 40 .mu.m or smaller are mixed in a proportion of several percent by weight in the whole particles thereby producing dust. This method is unable to reduce the average particle size to 150 .mu.m or smaller.
Method (3) allows, by sphering, control of the particle size distribution and average particle size in a particle size range of 100 .mu.m and greater. This control is however difficult for particle sizes smaller than 100 .mu.m. After the sphering treatment, it is necessary to treat a large amount of solvent used. Further, spherical latex particles are coagulated externally so that their coagulation does not take place uniformly, resulting in a potential problem of fish eyes upon processing of the polymer. Method (4) is used to produce a particulate polymer having a narrow particle size distribution. It is however unable to control the particle size as desired while retaining the narrow range of the particle size distribution. Although not so serious as in method (3), method (4) is also accompanied by the drawback that an organic liquid is added in an amount of 60-500 parts by weight per 100 parts by weight of the polymer and a large amount of the organic liquid must hence be treated. Method (5) is also used to produce a particulate polymer having a narrow particle size distribution. It is however unable to control the average particle size as desired while retaining the narrow range of the particle size distribution. Method (5) is also accompanied by the drawback that an organic liquid is added at a volume ratio of 1-5 relative to the volume of the polymer and a large amount of the organic liquid must hence be treated.
In addition, methods (4) and (5) include an unstable granulation step in that the apparent viscosity of the mixture rapidly increases for a while after the addition of the organic liquid in spite of the inclusion of a suspension stabilizer and surfactant because the addition of the organic liquid is batchwise. Accordingly, the particle size distribution of particles to be produced is significantly affected by the revolution speed of the stirrer, the shape of the stirrer, the amount of the organic liquid used, etc., thereby making it difficult to produce, in a large volume and at a low cost, particles with a controlled particle size distribution and bulk specific gravity. Batchwise granulation shows a different granulation behavior from continuous granulation in which an organic liquid is fed continuously. Batchwise granulation may be described as a different technique from continuous granulation.
Method (6) permits easy formation of a spherical particulate material because it is identical in mechanism to spray drying. It is however accompanied by the drawbacks that there is a limitation to the size of particles and that a large apparatus is required due to the coagulation taking place in a vapor phase.
On the other hand, the automation of metering of powder and the increased size of storage and transportation facilities are being employed. From the standpoint of avoiding the caking powder particles during storage or the clogging of transportation lines due to insufficient flowability of powder, there is a strong demand for the development of a granular polymer which is easy to handle. Resin have recently been required to have special functions. Since this demand for special functions may not be met by a single polymer in many instances, the polymer is often used together with one or more other polymers and one or more modifiers. In this case, it is necessary to control their particle sizes suitably so as to avoid variations in composition due to segregation of the particulate materials in the course of storage or transportation.
Furthermore, when used as a mixture with one or more other particulate materials, the particulate polymer is also required to have a suitable particle size to avoid variations in composition due to segregation of the particulate polymer and/or materials in the course of storage and/or transportation. There is hence a strong demand for the development of a continuous production process for a particulate polymer, which permits control of the bulk specific gravity and the particle size distribution of the particulate polymer.
When a polymer is used in the form of a mixture with one or more other materials, on the other hand, the uniformity of the mixture is enhanced by melting, mixing and kneading them in an extruder or the like. For this application, it is necessary to make the particulate polymer as small as possible so as to reduce as much as possible the proportion of particles of 40 .mu.m or smaller, which adversely affect the handling property of the particulate polymer, and at the same time to facilitate its dispersion upon mixing. There is a strong desire to develop a method for controlling the particle size of a particulate polymer, allowing control of the average particle size while maintaining the particle size distribution narrow as described above